Improvement to the linera-views-derive code (#4421)

## Motivation

The code in `lib.rs` has several failure scenarios that result in
crashes, panics, or other errors.

Fixes #744 

## Proposal

It is completely possible to implement views for unnamed structs, that
is:
```rust
pub struct CounterState(pub RegisterView<u64>);
```

However, we choose to make this illegal. The reason is that this kind of
structs would not be used in the protocol code and it is also not
allowed by `async_graphql::SimpleObject`.

What this PR does is to add several error patterns without panic to the
macro code:
* Unnamed structs lead to explicit error.
* Track the error produced by `RegisterView<u64>` which can be produced
by missing Context or by missing view macro.
* structs must have at least one entry.

## Test Plan

The CI.

## Release Plan

- Nothing to do / These changes follow the usual release cycle.

## Links

None.

Author: MathieuDutSik